The invention concerns a facility for the production of isobutene and propylene from a C4 cut.
Steam cracking feeds constituted by light paraffinic cuts produces ethylene and propylene which are required for petrochemistry. It also produces a certain number of other heavier products, in particular a C4 hydrocarbon cut which principally contains 1,3-butadiene, isobutene, n-butenes and butanes, accompanied by traces of acetylenic hydrocarbons.
In addition to gasoline and gas oil as the principal products, catalytic cracking of heavy hydrocarbon feeds produces lighter products, among them a C4 hydrocarbon cut which contains principally isobutane, isobutene, n-butenes and butanes, accompanied by small quantities of 1,3-butadiene and acetylenic hydrocarbons.
Until recently, only 1,3-butadiene and isobutene were of use in the polymer industry, in particular the tire industry for the former. An increase in the longevity of tires and a relative stagnation in demand have meant that there is now a surplus of butadiene which is not used or is not used properly. In contrast, there has been a reawakening in interest in isobutene which can be used, for example, for the synthesis of ethers for use as additives in automobile fuels or as a monomer for the synthesis of polyisobutene.
The present invention proposes a facility for the treatment of a C4 hydrocarbon cut containing principally isobutene, n-butenes, butanes, and 1,3-butadiene in various quantities, which includes separating the isobutene by distillation and which can transform the 1,3-butadiene and n-butenes to propylene which can, for example, be used for polymerisation.
The relative proportions of ethylene and propylene produced in a steam cracking operation can be modulated to a certain extent by changing the nature of the feed and modifying the operating conditions (severity) of cracking. However, an operative mode which is oriented towards producing a larger proportion of propylene inevitably entrains a reduction in the ethylene yield and higher production of the C4 cut and gasoline fraction.
A further aim of the present invention is to increase the production of propylene while maintaining a high yield of ethylene by treating the C4 hydrocarbon cut and thus without the need for a reduction in the severity of the steam cracker.
In Applicants"" issued patent application, now U.S. Pat. No. 6,075,173 issued Jun. 13, 2000, the process aspect of the invention was elected for the conversion of an olefinic C4 cut to isobutene and propylene, the cut containing diolefins, butene-1, butene-2, isobutene and acetylenic impurities, the process comprising the following successive steps:
1) selective hydrogenation of diolefins and acetylenic impurities with isomerisation of butene-1 to butene-2 to obtain an effluent containing mainly butene-2 and isobutene, and containing practically no diolefins or acetylenic compounds, preferably by passing the cut in a liquid phase over a catalyst comprising at least one metal selected from the group formed by nickel, palladium and platinum, deposited on a support, at a temperature of 20-200xc2x0 C., a pressure of 0.1-5 MPa, a space velocity of 0.5-10 hxe2x88x921, and with an H2/diolefin (molar) ratio of 0.5 to 5, preferably 1 to 3;
2) separating, by distillation, an overhead cut containing mainly isobutene and butene-1 which was not converted during the first step, and a bottom cut containing essentially butene-2 and butane, at most 1% by weight of isobutene and at most 1% by weight of butene-1;
3) carrying out metathesis of the butene-2 cut from the preceding step with ethylene, in the presence of a catalyst comprising at least one rhenium oxide deposited on a support, at a temperature in the range 0xc2x0 C. to 100xc2x0 C., and at a pressure which is at least equal to the vapour tension of the reaction mixture at the reaction temperature, to obtain an effluent containing propylene, metathesis being followed by separation of the propylene.